Off-Normal CO2 Desorption from the Photooxidation of CO on

Aug 6, 2010 - Nikolay G. PetrikRentao MuArjun DahalZhitao WangIgor .... Mikhaylov , Konstantin V. Nikitin , Nadezhda I. Glazkova , Vyacheslav N. Kuzne...
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Off-Normal CO2 Desorption from the Photooxidation of CO on Reduced TiO2(110) Nikolay G. Petrik* and Greg A. Kimmel* Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352

ABSTRACT Photoinduced reactions between O2 and CO on reduced rutile TiO2(110) are studied at low temperature (∼30 K). Photon-stimulated desorption (PSD) of O2, CO2, and CO is observed with comparable yields. Isotope labeling experiments indicate that O2 chemisorbed in a vacancy is more active for photooxidation than O2 chemisorbed on a Ti5c site. The angular distribution for the desorbing CO2 is peaked at ∼40° with respect to the surface normal in the [110] azimuth (i.e., perpendicular to the bridging oxygen rows), suggesting that CO2 is produced from O2 occupying an oxygen vacancy and CO adsorbed on a Ti5c site next to it. The experimental results are consistent with CO2 being produced from a transition state that has been predicted theoretically. The CO PSD from TiO2(110) is enhanced dramatically by the presence of chemisorbed O2, suggesting that it is a byproduct of the CO photooxidation process. SECTION Surfaces, Interfaces, Catalysis

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an O2 adsorbed in a bridging oxygen vacancy, VO, has a greater reaction probability than an O2 chemisorbed on an adjacent Ti5c site. Hydroxylating the surface, which converts each VO into two bridging hydroxyl groups, quenches the CO photooxidation reaction. The results suggest that CO2 is produced from chemisorbed O2 residing in an oxygen vacancy and CO adsorbed on a Ti site next to it via a transition state that has been predicted theoretically. These results significantly advance our understanding of this prototypical oxidation reaction on TiO2. Figure 1 shows the 13C16O, 16O2, and 13C16O2 PSD signals at 30 K versus time for several experiments on an annealed TiO2(110) surface with a vacancy concentration, θ(VO), of ∼0.05 ML. For these experiments, the quadrupole mass spectrometer (QMS) was oriented such that it did not have a direct line-of-sight to the front face of the sample. In this configuration, the PSD signals are proportional to the total (angle-integrated) desorption rate. In the first experiment (Figure 1a), 0.17 ML of 16O2 was dosed at 30 K and then briefly annealed at 100 K to desorb any physisorbed O2 and leave the oxygen vacancies saturated with chemisorbed O2 (θsat ≈ 0.10 ML).16,26 Next, 1 ML of 13C16O was adsorbed, and the surface was then irradiated with UV photons at 30 K. As reported previously,5,7 photon-stimulated reactions between 16 O2 and 13C16O produce 13C16O2 (Figure 1a, blue line), which desorbs along with 16O2 (Figure 1a, red line). Although not previously reported, PSD of 13C16O is also seen with a signal that is comparable to the other two products (Figure 1a, black line). For all of the products, the PSD signals have a prompt

hoton-stimulated chemistry on the surfaces of TiO2 is very important for a broad spectrum of practical applications, including the photooxidation of organic pollutants, self-cleaning surfaces, solar energy production, and nonthermal catalysis.1-4 As a result, the thermal and photon-stimulated reactions on various TiO2 surfaces have been extensively studied, and TiO2 has become a system of fundamental importance for the general understanding of oxide surface chemistry.2-18 The reactions between CO and O2 have been studied in detail on metals,19-24 and it is a model system for studying the mechanisms of thermal oxidation and photooxidation on TiO2. Yates and co-workers investigated the photooxidation of CO on TiO2(110) at ∼100 K and found that UV irradiation led to either photodesorption of chemisorbed O2 or photooxidation of CO to CO2.5,7,25 The energy thresholds for both O2 and CO2 photon-stimulated desorption (PSD) were at ∼3.1 eV, corresponding to the band gap of TiO2. This threshold indicates that the photochemistry is initiated by the generation of electron-hole pairs in the substrate. On the basis of thermal annealing experiments, they proposed the existence of two forms of chemisorbed O2, one form that photodesorbs as O2 and another which photooxidizes CO. However, the mechanisms of CO photooxidation on TiO2(110) are not well understood. For example, whether the photooxidation reaction involves the photogenerated holes or electrons (or both) is unclear. In this Letter, we investigate the photochemical reactions between O2 and CO on rutile TiO2(110) at ∼30 K. The main PSD products are O2, CO2, and CO. The angular distribution of the photodesorbed CO2 has a pronounced peak at ∼40° with respect to the surface normal in the [110] azimuth (i.e., CO2 desorbs in a plane perpendicular to the bridging oxygen rows). Isotope labeling of the chemisorbed O2 indicates that

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Received Date: June 29, 2010 Accepted Date: August 2, 2010 Published on Web Date: August 06, 2010

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Figure 2. CO2 PSD signals versus time for (a) θ(18O2) = 0.5θsat dosed first, θ(16O2)=0.5θsat dosed second, and then 1 ML of CO; (b) θ(CO) ≈ 1 ML on oxidized TiO2(110); (c) physisorbed O2 and CO2 on oxidized TiO2(110); and (d) θ(O2) = θsat and θ(CO) ≈ 1 ML on hydroxylated TiO2(110). The curves are displaced for clarity.

Figure 1. 13C16O (black lines), 16O2 (red line), and 13C16O2 (blue lines) PSD signals versus time. (a) For θ(16O2)=θsat and θ(13C16O) ≈ 1 ML, the 13C16O and 13C16O2 PSD signals are comparable. (b) For θ(16O2)=0.5θsat and θ(13C16O) ≈ 1 ML, the initial 13C16O2 signal is comparable to (a) but decays more quickly. (c) The 13CO and 13 16 C O2 PSD signals from 1 ML of 13C16O without predosed 16O2 are very small. The curves are displaced for clarity.

between the oxygen adsorbed in a vacancy and that on a Ti5c site (during adsorption and/or irradiation) cannot be ruled out, it is possible that O2 adsorbed in the vacancies is responsible for all of the photooxidation. Previous research has shown that the lattice oxygen in TiO2 is not involved in the photooxidation and that CO2 is produced using the CO and one atom from an O2.7 The results in Figure 2b show that oxygen adatoms do not photooxidize CO to CO2 (see also ref 7). In this experiment, TiO2(110) was oxidized by exposing it to ∼8  1015 18O2/cm2 at 300 K. Oxygen is known to dissociatively adsorb O2 at 300 K, healing VO's and leaving oxygen adatoms on nearby Ti5c sites.14,30 The oxidized surface was then dosed with ∼1 ML of CO. When irradiated with UV photons, essentially no CO2 PSD was observed (Figure 2b). Physisorbed O2 also does not photooxidize CO. After oxidation at 300 K, only a small amount of O2 chemisorbs at 30 K,16 and any additional O2 physisorbs (i.e., adsorbs as a neutral O2 that is bound by electrostatic and dispersion interactions). For 0.1 ML of 18O2 physisorbed on a surface oxidized along with 1 ML of CO, the CO2 PSD yield is quite low (Figure 2c), and the residual signal can be accounted for by the small amount of O2 that chemisorbs on this surface.16 Therefore, O2 must be chemisorbed in order to photooxidize CO, and O2 chemisorbed in the vacancy is the most reactive. Since one atom from molecularly adsorbed O2 is used to photooxidize CO, the oxygen atom from the O2 that is not involved in the photooxidation reaction should be left adsorbed on the surface. This provides a plausible explanation for why the O2 chemisorbed in the vacancy is more reactive; it should be energetically more favorable for the remaining oxygen atom to adsorb in the original oxygen vacancy (thus eliminating the vacancy)

initial rise and a complicated nonexponential decay. The 16O2 PSD decays the fastest, while the 13C16O and 13C16O2 kinetics are similar. Since only a very small 13C16O (Figure 1c, black line) and no 13C16O2 (Figure 1c, blue line) PSD signals are observed from 1 ML of 13C16O without chemisorbed 16O2, the 13 16 C O PSD seen when both 16O2 and 13C16O are adsorbed is associated with the photooxidation reaction. This conclusion is also supported by the angular distributions of the desorbing CO and CO2 discussed below. At saturation, two O2 chemisorb per oxygen vacancy on reduced TiO2(110) (i.e., θsat ≈ 2θ(VO)).16,26 On the basis of density functional theory (DFT), the first molecule most likely adsorbs in the vacancy parallel to the surface and perpendicular to the bridging oxygen rows,10,27,28 and the second molecule occupies a neighboring Ti5c site.10 Therefore, in addition to experiments with θ(16O2) = θsat, we also investigated the production of CO2 when θ(16O2) = 0.5θsat and θ(13C16O) = 1 ML (Figure 1b, blue line). The initial 13C16O2 PSD signals are comparable for both θ(16O2) = θsat (Figure 1a) and θ(16O2) = 0.5θsat (Figure 1b). However, the signal decays more quickly for the smaller 16O2 coverage. The results in Figure 1 for θ(16O2) ≈ 2θ(VO) and θ(16O2) ≈ θ(VO) suggest that an O2 residing in a vacancy is more reactive for CO photooxidation than an O2 adjacent to a vacancy. To probe the contribution of each chemisorbed O2 in the CO2 production in more detail, we investigated the reactions with coadsorbed 16O2 and 18O2. Figure 2a shows the C16O2 (black line) and C16O18O (red line) PSD versus time for 18O2 dosed first and 16O2 dosed second with θ(O2) = θ(VO) ≈ 0.05 ML for each isotope. The sample was briefly annealed at 100 K after each O2 dose, and then, 1 ML of C16O was dosed at 30 K. For this experiment, 18O2 should be primarily chemisorbed in the vacancies, while 16O2 should be chemisorbed on adjacent Ti5c sites.10 During irradiation, the C16O18O PSD signal is initially ∼3 times larger than the C16O2 PSD signal.29 Reversing the order of the 16O2 and 18O2 doses resulted in a larger C16O2 PSD yield (data not shown). Therefore, the result in Figure 2a suggests that the O2 occupying the vacancy (i.e., dosed first) is primarily involved in the CO photooxidation. Since exchange

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VO þ O2 ðvacÞ þ CO f CO2 þ Ob

ð1Þ

versus a reaction which leaves the extra oxygen atom adsorbed on a Ti5c site O2 ðTiÞ þ CO f CO2 þ Oad

ð2Þ

On the basis of this hypothesis, we might expect that if the oxygen atom is blocked from occupying the vacancy, the

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Figure 4. Schematic of CO photooxidation on TiO2(110), showing “Top” (upper portion) and “Side” (lower portion) views of the reaction. The [110] and [001] azimuths are marked in (a). Lattice oxygen and titanium are represented by small red and gray spheres, respectively. Oxygen and carbon atoms in the CO and O2 are shown as larger green and black spheres, respectively. (a) Initial state: O2 chemisorbed in the vacancy and CO adsorbed at an adjacent Ti5c site. (b) Proposed transition state (see refs 9 and 37). (c) Off-normal CO2 desorption in the [110] direction. (d) Perspective view. Note that the experiments only measured the angular distributions of the CO2 in two azimuths. However, on the basis of the symmetry of the crystal, we expect the CO2 to desorb in two lobes, as shown.

thus providing valuable information about the reaction mechanism.33-35 Figure 3 shows the time-integrated O2, CO, and CO2 PSD yields versus the polar angle of the QMS, θdes, relative to the surface normal for two different azimuthal orientations of the TiO2(110) (see Supporting Information, Figure S1).36 For these experiments, θ(VO) = 0.08 ( 0.01 ML, and surfaces with θ(O2) = θsat and θ(CO) = 1 ML were prepared (as described for Figure 1) and irradiated at 30 K. The O2 PSD signal is narrow and peaked at normal for both azimuthal orientations (Figure 3a). For desorption parallel to the [001] azimuth (i.e., in a plane parallel to the bridging oxygen rows), the CO and CO2 PSD signals are also peaked at normal incidence and narrow compared to a cosine distribution (Figure 3b and c, black symbols). In contrast to the O2, the CO and CO2 PSD signals are peaked at θdes ≈ 50 and 40°, respectively, for desorption in the [110] direction (Figure 3b and c, red symbols). The detector response to a delta function signal centered at θdes = 40° and φdes = 0 (see Supporting Information) is also shown for comparison (Figure 3c, green line). Due to the relatively large angular acceptance of the detector, the actual PSD distributions will have a smaller angular width than that seen in the data. The noncosine angular distributions in Figure 3 indicate that the O2, CO, and CO2 have not accommodated to the surface temperature prior to desorption, that is, desorption results from a nonthermal, bond-breaking process without further (appreciable) scattering from the surface. Thus, for CO2, the results indicate that the bond between the CO2 and the remaining oxygen atom in the transition state is perpendicular to the rows of bridging oxygen atoms and tilted ∼40° with respect to the surface normal (see Figure 4). The thermal reaction of CO with an O2 in an oxygen vacancy on TiO2(110) has been examined with DFT.9,37 The calculations suggest that CO2 is produced from a transition state that lies in a plane parallel to the [110] azimuth and normal to the surface (see Figure 4). In the calculated transition state, the O2 and CO tilt toward each other such that the molecular axes are no longer parallel and perpendicular to the surface, respectively (Figure 4b). The calculated transition

Figure 3. Integrated (a) O2, (b) CO, and (c) CO2 PSD yields versus the desorption angle, θdes, relative to the surface normal for two azimuthal orientations of the TiO2(110) for θ(O2) = θsat and θ(CO) ≈ 1 ML. The red and black symbols correspond to desorption in the plane perpendicular and parallel to the bridging oxygen rows, respectively (i.e., in the [110] and [001] azimuths; see Figure 4). The data are normalized to the angle-integrated yields (i.e., at θdes > 90°). For reference, thermal [i.e., cos(θdes)] distributions (blue dotted lines) are shown.

photooxidation of CO would be reduced or eliminated. To test this, we examined the photooxidation of CO on hydroxylated TiO2(11) (Figure 2d). Water dissociates in VO's on TiO2(110) above ∼200 K,8,30 creating two bridge-bonded hydroxyls, OHb, and eliminating the vacancy (i.e., VO þ H2O f 2OHb). However, the electronic defects associated with the vacancy (Ti3þ states) remain largely unperturbed,15,31,32 and both surfaces can chemisorb the same amount of O2.16 Therefore, the hydroxylated surface is a good candidate for testing the relative importance of reactions 1 and 2. For this experiment, reduced TiO2(110) was hydroxylated by exposing it to 1 ML of water at 400 K. The hydroxylated surface was dosed sequentially with O2 and CO (θ(O2) = θsat and θ(CO) = 1 ML) and then irradiated. Since very little CO2 is produced (Figure 2d) and the CO PSD is also suppressed (not shown), this experiment suggests that having a vacancy available to accept the remaining oxygen atom is important. For CO photooxidation, the results presented in Figure 2 indicate that CO2 is primarily produced from an O2 residing in a vacancy and a CO molecule occupying an adjacent Ti site. Thus, we expect that the bond formed between the reactants in the transition state is perpendicular to the rows of bridging oxygen atoms on the surface. For thermal and nonthermal reactions, the bonding geometry in the transition state can influence the angular distribution of the desorbing products,

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state, which is asymmetric relative to the original vacancy site, is consistent with the off-normal desorption of CO2 observed in our experiments. In particular, since the bond between the CO2 and the remaining oxygen atom is not normal to the surface, we should expect off-normal desorption, as observed (Figure 3). Some other aspects of the calculated reaction also fit the observations. In particular, the reaction uses one atom from an O2 molecularly adsorbed in a vacancy, and the other atom remains in the vacancy (healing it). On the basis of our results and the transition state proposed in the literature, Figure 4 illustrates the proposed steps leading to the offnormal CO2 PSD.9,37 The angular distribution for the CO PSD (Figure 3b), which is qualitatively similar to the CO2 distribution, suggests that some of the CO desorption occurs when the transition state decays back to the reactants (i.e., CO þ O2) instead of reaching the product state. The CO2 and CO PSD signals versus time are also similar, again suggesting that they represent two different reaction channels with a common precursor. The decay of the transition state back to the reactants could potentially lead to exchange of oxygen atoms between the CO and the O2. (e.g., C18O þ 16O2 f 18O-C16 O-16O f C16O þ 16O18O). However, we did not observe any such exchange. The calculated barrier to create the transition state is ∼0.4 eV, which exceeds the CO binding energy on TiO2(110).9,37 This barrier potentially explains why thermal oxidation of CO on O2-TiO2(110) is not observed experimentally.7 However, the DFT calculations also predict a barrierless reaction between CO and an O adatom (produced, for example, as result of O2 thermal dissociation in the vacancy above 150 K).9 In contrast, no thermal or photon-stimulated reactions between CO and oxygen adatoms are observed experimentally. Furthermore, the transition state for the CO photooxidation reaction is produced via electronic excitations,38 while the calculated transition state is for the ground electronic state. Therefore, while the calculated transition state has the correct symmetry to match the observed angular distributions, caution is required when comparing the theory and experiment. CO photooxidation is a substrate-mediated process involving reactions of the adsorbed molecules with electrons and/ or holes produced within the TiO2.7 The observation that the CO PSD yield is small without predosed O2 suggests that CO does not interact efficiently with the photogenerated electrons or holes. Thus, the results presented here suggest that the photooxidation reaction is initiated by the interaction of electrons and/or holes with O2 chemisorbed in bridging oxygen vacancies. However, whether the reaction is primarily hole-mediated or electron-mediated is unclear. For example, for a single O2 chemisorbed in a (neutral) vacancy as Oδ2 , the (2-δ)f CO2 þ O2reaction would be CO þ Oδ2 þ VO b . Since the oxygen atom that heals the vacancy (i.e., the O2b ) takes all of the charge originally associated with the vacancy, no charge transfer is required. This is consistent with the DFT calculations indicating that the highest barrier in the CO oxidation is associated with changes in the bonding configuration of the O2 in the vacancy, not with changes in its charge.37 One possibility is that the photooxidation reaction involves the sequential reaction of a hole and an electron (or vice versa),

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leaving the net charge unchanged. Another possibility is that the charge of a vacancy is not -2e, as has been recently suggested.39 However, more research is needed to obtain a comprehensive understanding of this reaction. In summary, we have investigated the photoinduced reactions between O2 and CO on reduced TiO2(110). Photostimulated desorption of O2 and CO and photon-stimulated production of CO2 are observed with comparable yields. CO2 is mainly a product of reactions between the O2 chemisorbed in an oxygen vacancy and a CO adsorbed on a Ti site next to it. The angular distribution of the photodesorbed CO2 is noncosine, narrow, and off-normal; it peaks at ∼40° with respect to the surface normal in the [110] azimuth. The results are consistent with CO2 being produced from a transition state that has been predicted theoretically. CO PSD from TiO2(110) is enhanced by the presence of chemisorbed O2 and also has off-normal desorption, suggesting that CO desorption is a related to the CO photooxidation process.

EXPERIMENTAL SECTION The experiments were performed in a UHV system that has been described previously.40 The 10  10  1 mm rutile TiO2(110) crystals (CrysTec GmbH or Princeton Scientific) were repeatedly sputtered with 2 keV Neþ ions and annealed at 950 K. For the ion-sputtered and annealed surfaces, samples with oxygen vacancy concentrations, θ(VO), ranging from 0.05 to 0.08 ML were used. For the results in Figures 1 and 2, θ(VO) = 0.05 ( 0.01 ML, but similar results were obtained for a sample with θ(VO) = 0.08 ML. Gases were dosed at the base temperature (typically ∼30 K) where the sticking coefficients were high (∼0.8 for both O2 and CO).26,41 Saturation coverages of chemisorbed oxygen, θsat, were prepared by adsorbing more O2 than the saturation coverage and then briefly heating the sample to 100 K to desorb any physisorbed O2. Gas exposures were performed with a molecular beam at normal incidence to the surface. Photon irradiations were performed using a 100 W Hg lamp (Oriel #6281). The entire UV portion of the lamp's spectrum was used during photon irradiation, but a water filter was used to eliminate the infrared. During photon irradiation, the increase in the sample temperature was